Electrophotographic methods using electrostatic developing are well known. Such methods utilize conductive developers, e.g., British Pat. No. 1,567,219 and U.S. Pat. No. 4,060,451. The latter patent discloses a thin zinc oxide photoconductive layer using a one component toner having a resistivity below 10.sup.5 ohm.m. See also British Pat. No. 1,406,983 relating to a one component toner powder.
In addition to zinc oxide, selenium, in the form of amorphous selenium, is used as a photoconductive layer on rotating drums. In other photosensitive devices, other types of electrically conductive substrates such as amorphous silicon and silicon-germanium have been used. See U.S. Pat. No. 4,451,546. However, such materials have not found application in electrophotography.
In U.S. Pat. No. 4,225,222 there is disclosed a process for producing an amorphous silicon layer on a drum which is from 10 to 100 .mu.m thick. The advantage of this silicon layer is that it has considerable resistance to wear. However, it has the significant disadvantage that the dark-decay rate is too high for practical applications. Consequently, it is an object of the present invention to obviate the disadvantage caused by a high dark decay rate or a fast dark decay time.
To reduce the dark decay rate and increase the dark decay time it has been proposed to provide a thick silicon layer with a thin top layer of silicon nitride or silicon carbide. Although a top layer of this kind has some beneficial effect, it is not sufficient to eliminate the problem of a high dark decay rate or an excessively fast dark decay time. The major benefit of such a layer is to increase the surface hardness of the silicon layer thereby improving its wear resistance, such as described in Japanese Patent Application No. 57-200047.
U.S. Pat. No. 4,297,392 discloses a method of producing a electrophotographic element having a thin film of amorphous silicon under specific conditions. While a broad range of thicknesses for the film layer is mentioned, there is no statement as to the actual dark decay rate of the film layer and no indication that very thin layers, such as between 0.5 and 3 .mu.m, have a high dark decay time, namely greater than 25 seconds, compared to thicker layers. Moreover, the proposed increase in the time constant or surface potential decay, mentioned in the patent, is only achieved by the use of a separate insulating layer. See FIG. 3 and Column 5, lines 20-26.